Xen Performance Guide

Disk utilisation is the biggest bottleneck in virtualisation. The best way to install the guest OS (DomU) is also not an obvious one. Most people have a RAID subsystem as storage for all the DomU virtual disks – but having this misaligned or not optimised can lead to significant performance degradation. This mostly comes about when having an LVM scheme where you use an LV to store the DomUs filesystem – but you use the ISO installer – so you end up offsetting everything by the partition table + boot data. This means in most cases, when you write a block to the DomU’s filesystem, it translates to two block writes in the physical storage array – you’re essentially writing twice as much data as you need.

So, my way to solve this was to use the LV directly – with no partition table. We also start the installation via the Dom0 shell – that way the tools have full visibility as to the layout of the underlying block storage.

This guide focuses on EL6 – specifically Scientific Linux 6. You can probably substitute easily enough for CentOS or RHEL.